This invention relates generally to thermosetting films and, more particularly, to a die-cuttable, B-stageable, thermosetting ferrite/epoxy film which is useful as a magnetic field keeper in a plated wire memory system.
As is known, a plated wire memory comprises a plurality of word straps, typically conductive stripes affixed to a substrate, which are crossed by a multiplicity of plated wires. Because the word straps extend typically 1-2 mils above the surface of the substrate and run in parallel and in close proximity to one another for considerable distances, there tends to be strong interaction among the magnetic fields produced by the currents which are passed through these conductors. As such, it is known to overcoat and fill the channels between the word straps with a material including ferrite to "keep" the magnetic fields, thereby limiting the magnetic field interactions between the word straps.
In the prior art, the method for providing a keeper over the plated wire memory word straps has included providing a ferrite/epoxy composition having sprayable viscosity, and spraying a layer of the composition over the word straps. One such prior art compostion is PC-17, sold by Hysol Co., Olean, N. Y. PC-17 is a bisphenol A-based epoxy resin with a amide curative. Its viscosity starts to increase at room temperature in less than one hour. This relatively short pot life requires that the composition be used immediately, and generally results in a substantial waste factor since excess mixed material cannot be salvaged.
Another drawback of this prior art keeper using an amide cured epoxy lies in the requirement that it must be sprayed onto the word straps. Spray application requires a high degree of skill by the operator in order to ensure a void-free coating. This labor-intensive process also includes masking the areas which are to remain uncoated, sanding the cured outer surface to leave a uniform coating thickness, and removing the excess cured keeper from the borders of the word strap windows.
By way of summary, this prior art method has proven to be complex, unreliable, expensive and requires a high level of skill, and when a spraying error occurs, the entire memory unit may have to be discarded.